It is widely believed in the industry that certain levels of oxidative and thermal stability in lubricant oils can only be obtained by using full synthetic formulations. It is an object of the present invention to obtain performance similar to a full synthetic formulation using mineral base oils in order to recognize the significant cost difference between expensive synthetic base oils and less expensive mineral base oils.
In a paper by O'Connor et al., entitled Axle Efficiency--Response to Synthetic Lubricant Components (SAE Paper No. 821181), the authors state that "[i]nvestigations with both partial- and full-synthetic base formulations have shown improvements compared to conventional petroleum base gear oils. Maximum benefits are gained with total synthetic base type formulations." This paper fails to teach the advantages obtained by the mineral based lubricating oil formulations of the present invention.
U.S. Pat. No. 4,758,364 is directed to an automatic transmission fluid comprising either a mineral or synthetic oil, C.sub.2 -C.sub.10 monoolefin polymers, and methacrylic acid ester copolymers. This patent does not teach or suggest the specific mineral oil of the present invention.
U.S. Pat. No. 4,853,139 is directed to lubricating oil compositions comprising a base oil having a kinematic viscosity at 100.degree. C. of 1.5 to 50 cSt, a pour point of -25.degree. C. or lower and a viscosity index of at least 60; an ethylene-.alpha.-olefin copolymer having a number average molecular weight of 1,000 to 8,000; and at least one additive selected from an extreme pressure agent, an anti-wear agent, an oiliness agent and a detergent dispersant. This reference fails to teach the specific mineral oils of the present invention.
EP 0281060 B1 is directed to lubricating oil compositions for traction drives comprising a specific base oil; an ethylene-.alpha.-olefin copolymer having a number average molecular weight of 800 to 8,000; a polymethacrylate having a number average molecular weight of 10,000 to 100,000; and an anti-wear agent. This reference fails to teach the specific mineral oils of the present invention.
EP 0790294 A2 is directed to lubricating oil compositions comprising a base oil; 5 to 30 wt % of at least one polymer having a weight average molecular weight of less than 10,000; and 2 to 12 wt % of a polymer having a weight average molecular weight of greater than about 15,000. This reference fails to teach the specific mineral oils of the present invention or recognize the benefits obtained by the specific combinations of the present invention.